The present invention relates to a process for preparing sorbic acid, and more particularly to the preparation of sorbic acid from .gamma.-vinyl-.gamma.-butyrolactone with novel catalysts.
Sorbic acid is industrially manufactured by reacting crotonaldehyde with ketene to give a polyester which is formed through an intermediately produced .beta.-lactone and converting the polyester to sorbic acid by heating or by means of acid, base or ion-exchange resin. However, such a process is not necessarily advantageous in point of its procedures or economy because the handling of polyester and the recovery or purification of sorbic acid are troublesome and also because it requires complicated process control.
Two of the present inventors, Masahiko Miyashita and Tokio Hashimoto developed the novel process for preparing sorbic acid by contacting .gamma.-vinyl-.gamma.-butyrolactone with an acid selected from the group consisting of mineral acids, aromatic sulfonic acids, aliphatic sulfonic acids and halogenated carboxylic acids or an acidic cation-exchange resin at an elevated temperature (as shown in U.S. patent application Ser. No. 570,515 filed on Apr. 22, 1975, now U.S. Pat. No. 4,022,822). However, in case of using a cation-exchange resin as a catalyst for cleaving the .gamma.-lactone, a relatively large amount of tarry materials adheres to the catalyst during the use for a long period of time, and it is impossible to use for a long period of time or to reuse such a catalyst because the yield of sorbic acid lowers. Therefore, it is necessary to revive the activity thereof, but very large quantities of a solvent is required for reactivating the catalyst so as to possess a sufficient activity. Also, some of the proposed acid catalysts, for instance, mineral acids, are liquid and, therefore, they cannot be used in the reaction in the vapor phase as they are. Further, the use of a liquid acid such as a mineral acid as a catalyst requires much labor, for instance, the crystals of sorbic acid recovered from the reaction mixture must be sufficiently washed to remove the acid since the acid adheres to the surfaces of the produced crystals of sorbic acid and runs the risk of lowering the qualities of sorbic acid, such as coloring and stability.